Yahoo calendar sync on one iPhone X but not another

My wife and I share a Yahoo calendar that usually syncs seamlessly across our two iPhone X's and our three Macs. However, my wife's iPhone X keeps losing the connection and can't receive updates or have her updates from iPhone sync back to our Yahoo account. A few weeks ago, I removed her Yahoo account from her phone and re-added it, and it worked for a few weeks but has lost it's syncing connection again. This has happened intermittently over the last couple of years on her phone but mine has never had any problems (nor have our Macs). Both of our phones have the latest software update. Help! If we can't get this resolved, I'm looking to migrate to a new calendar service and just hope the same thing doesn't keep happening.